Niall of the Nine Hostages and Dahut of Ys are legends. Poul and Karen Anderson bring them together and make them major characters in a novel. Dahut, like Harald Hardrada (see also here), envisages an Empire of the North. (Dahut, Chapter XIX, section 2, p. 425)
Maeloch has gone to spy in Hivernia but it is too late. Niall is in Ys with Dahut. The Ysan Gods pound the city with Their sea. All that remains is for Niall to deceive Dahut and persuade her to steal the King's Key to the sea gate. The legendary flooding of Ys is almost upon us and will happen at the end of Volume III and the beginning of Volume IV of the King of Ys Tetralogy.
Niall's lie to Dahut is that possession of the Key will empower him, Niall, to challenge Gratillonius and win. Thus is the deceiver deceived.